Metal-Oxide-Semiconductor (MOS) technology has been used widely. A MOS device can work in three regions including a linear region, a saturation region, and a sub-threshold region, depending on the gate voltage Vg and the source-drain voltage Vds. The sub-threshold region is a region where voltage Vg is smaller than the threshold voltage Vt. A parameter known as sub-threshold swing (SS) represents the easiness of switching the transistor current off, and is a factor in determining the speed of a MOS device. The sub-threshold swing can be expressed as a function of m*kT/q, where m is a parameter related to capacitance, k is the Boltzman constant, T is the absolute temperature, and q is the magnitude of the electrical charge on an electron. Previous studies have revealed that the sub-threshold swing of a typical MOS device has a limit of about 60 mV/decade at room temperature, which in turn sets a limit for further scaling of operational voltage VDD and threshold voltage Vt. This limitation is due to the diffusion transport mechanism of carriers. For this reason, existing MOS devices typically cannot switch faster than 60 mV/decade at room temperatures. The 60 mV/decade sub-threshold swing limit also applies to “Fin” Field Effect Transistor (FinFET) or ultra-thin-body MOSFET on silicon-on-insulator (SOI) devices. A FinFET comprises channels on the top surface and sidewalls of a fin. However, even with better gate control over the channel, an ultra-thin body MOSFET on SOI or FinFET can only achieve close to, but not below, the limit of 60 mV/decade. With such a limit, faster switching at low operational voltages for future nanometer devices cannot be achieved. To solve the above-discussed problem, Tunnel Field-Effect Transistors (TFETs) have been explored. In an existing MOSFET, the SS is limited by the diffusion of carriers over the source-to-channel barrier where the injection current is proportional to kT/q. Hence at room temperature, the SS is 60 mV/dec. In a TFET, injection is governed by the band-to-band tunneling from the valence band of the source to the conduction band of the channel. Since the current mechanism is tunneling determined, the current shows very weak temperature dependence, arising mainly due to band-gap changes with temperature. Accordingly, the SS is not limited by the temperature, and much lower SS can be achieved.
In TFETs, both the on-current and the off-current are determined by band-to-band tunneling from the valence band to the conduction band of the semiconductor material. Therefore, the on-current is generally limited by the band-bending in the channel. Various methods have been proposed to enhance the on-current, such as using a small band-gap source material to reduce tunneling barrier height and width, and also making tunnel FETs on narrow-band gap channel materials. Even though using narrow band-gap materials enhances the on-current exponentially, it has disadvantages. For example, the intrinsic carrier concentration in a semiconductor increases exponentially as a function of band-gap. The lower the band-gap, the higher the intrinsic carrier concentration at a given temperature is. This results in the higher off-state leakage currents.